1. Field of the Invention
This invention relates to a method for forming low resistance and low defect density contacts on a silicon semiconductor wafer. More particularly, the invention relates to forming a low resistance contact between a silicon semiconductor wafer and a first metallization layer through a high aspect ratio contact opening in an insulating layer.
2. Description of the Related Art
In the formation of integrated circuit structures, it is necessary to provide electrical contact to portions of the silicon semiconductor substrate surface through openings in an insulator formed over the substrate, e.g., to provide electrical contact to one of the electrodes of an MOS or a bipolar device. Conventionally, this is carried out by filling the contact opening with a conductive material such as aluminum. However, the use of a metal such as aluminum for filling contact openings has become unsatisfactory in some instances, particularly where the aspect ratios (height/width) of such contact openings has increased to ratios greater than one 1.
It has, therefore, been proposed to fill such contact openings with tungsten, due to its low resistance and ability to deposit conformally in contact openings, even when the opening has an aspect ratio of 1 or higher. However, tungsten does not make a good (low resistance) electrical contact with the surface of the silicon substrate beneath an insulator through which the contact opening is made, nor does the tungsten adhere very well to the sidewall surfaces of the contact opening through the insulator.
Several prior art approaches toward solutions to the problems which have arisen with the use of tungsten to fill the contact opening have been proposed. These prior art approaches include, first of all, the preliminary physical vapor deposition (PVD) of an intermediate titanium nitride layer on the walls of the opening through reactive sputtering of titanium nitride material from a titanium or titanium nitride target prior to filling the opening with tungsten. This formation of a titanium nitride layer over the insulator provides a chemical barrier which inhibits the subsequent tungsten CVD process from interacting with the underlying silicon substrate which interaction could result in tungsten encroachment into the contact causing large leakage current and non-functional devices.
This first approach requires a first or preliminary step of forming a silicide over and in the exposed silicon surface of the substrate before the titanium nitride deposition, since the titanium nitride does not form a good low resistance contact to silicon.
The use of this type of process is particularly objectionable, however, because of the generation of high levels of titanium nitride particles, resulting in a high density of defects in the integrated circuit structures on the wafer, which, in turn, leads to low yields.
It has also been proposed to physically deposit a titanium/tungsten (TiW) alloy or compound by reactive sputtering or other PVD methods, prior to filling the opening with tungsten. But this type of process has also been found to result in undesirable particle formation and deposition during the PVD formation of the titanium/tungsten alloy coating.
A third method has also been proposed wherein a layer of titanium is first deposited on the exposed silicon substrate and the insulator sidewalls by a PVD method such as sputtering followed by sputter deposition of a layer of titanium nitride. The structure is then annealed to cause the deposited titanium over the silicon to react with the silicon to form a silicide in and on the silicon surface, prior to filling the opening with tungsten.
However, when an annealing temperature of less than about 500.degree. C. is used, the type of silicide formed is not the desired low resistance type; while when a higher annealing temperature is used, silicon is found to migrate from the substrate to form titanium silicide along the walls of the contact opening, resulting in depletion of the silicon in the substrate adjacent the contact and, therefore, a non-functional device.
It would, therefore, be desirable to provide a process for the filling of a high aspect ratio contact opening in an insulator with tungsten, wherein a good electrical contact could be formed between the tungsten and the silicon substrate at the bottom of the contact opening, while providing good adherence of the tungsten to the surfaces of the insulator, without the formation of undesirable particles which characterizes prior art processes for filling such a contact opening with tungsten, and without the prior art migration of the silicon from the substrate to form titanium silicide along the walls of the contact opening which, in such prior art processes, resulted in depletion of the silicon in the substrate adjacent the contact.